This specification generally relates to voltage comparators. More specifically, the present specification describes method and apparatus for comparing input voltages having a small voltage difference.
Voltage comparators determine which of the two input voltages are larger or smaller. Since a voltage comparator often senses small differences between the input voltages and generates a digital output, a large amplification may be needed. The large amplification necessary for sensing small differences is constructed using a differential amplifier operating in a non-linear region. The differential amplifier has two transistors connected as a source-coupled pair with one of the transistors turned-off and the other transistor turned-on. Therefore, the amplifier has one transistor turned-on and drawing current even under static condition when the transistors are in a stable non-switching state.
As amplification circuits, comparators are susceptible to influence of noise on the input voltages. The noise on the input voltages causes erratic switching and false triggering of the comparator output. Thus, positive feedback can be applied to reduce the influence of noise on the comparison and to increase the flexibility of the differential threshold of switching. The flexibility of the threshold is increased by making the threshold less sensitive to the difference in input voltages and more sensitive to the previous levels of the input voltages. However, the feedback also tends to slow the response of the comparator and limits the lowest differential voltage which can be sensed.
The voltage comparators are used in various different applications such as in analog-to-digital converters (ADCs), signal generators, and image arrays and ADCs of complementary metal-oxide semiconductor (CMOS) active pixel sensors (APSs).
The voltage comparators used in the CMOS APSs are sensitive to a high fixed-pattern noise. The fixed-pattern noise is an unvarying display pattern resulting from the difficulty in exactly matching transistor thresholds on CMOS circuits for photocurrent amplification and transfer circuitry.